[Devel] [PATCH vz10 v2] ve/vtty: fix use-after-free on concurrent tty close and reopen

[Konstantin Khorenko]

Vasileios, Vladimir, please review the patch.

--
Best regards,

Konstantin Khorenko,
Virtuozzo Linux Kernel Team

On 6/30/26 18:06, Eva Kurchatova wrote:
> Two races in the vtty subsystem lead to use-after-free of tty_struct
> objects when vzctl console attach/detach cycles run concurrently with
> container-side tty open/close (e.g. SAK-triggered getty respawn):
> 
> Race 1: double-final between concurrent vtty master and slave close.
> 
> In tty_release(), the vttys (slave) side has o_tty == NULL because its
> driver subtype is PTY_TYPE_SLAVE, so the final-close check depends
> solely on !slave->count.  When master and slave close concurrently,
> both sides can independently determine final == true: the slave sees
> slave->count == 0, the master sees both counts == 0 (after the slave
> count underflows to -1 and gets reset).  Both then call
> tty_release_struct -> release_tty, and the second caller hits a
> use-after-free.
> 
> Fix this by adding a vtty-specific check after computing final: for
> vttys closes, also verify that the peer vttym count is not positive.
> This is safe without holding the vttym lock because the vttym side of
> tty_release holds tty_lock_slave(vttys) while decrementing vttym->count,
> so while we hold tty_lock(vttys) the vttym cannot have decremented yet.
> 
> Race 2: vtty_open_master reopens a dying tty pair.
> 
> After tty_release() sets final == true and releases tty_lock, there is
> a window before release_tty() runs under tty_mutex.  During this window
> vtty_open_master() can find the old vttym in the vtty map with count == 0,
> pass the ">= 1" check (which was designed as a "one vttym at a time"
> guard, not a liveness check), re-increment the counts, and hand out a
> file descriptor pointing to a tty_struct that is about to be freed.
> 
> Fix this by taking tty_lock(vttys) in vtty_open_master() to serialize
> with a concurrent tty_release() on the vttys side, so we read the slave
> count after any in-flight close has decremented it.  If vttys->count
> has reached zero the pair is dying, so return -EBUSY.  Incrementing
> vttys->count under the same tty_lock prevents a concurrent tty_release()
> from seeing zero and entering the final-close path.
> 
> The lock nesting pattern of tty_mutex -> tty_lock is preserved, which
> prevents any kind of A->B, B->A circular locking dependency.
> 
> Fixes: dfe187803cf9 ("ve/vtty: Don't close unread master peer if slave is nonzero")
> Signed-off-by: Eva Kurchatova <eva.kurchatova at virtuozzo.com>
> ---
>  drivers/tty/pty.c    | 31 ++++++++++++++++++++++++++-----
>  drivers/tty/tty_io.c | 17 +++++++++++++++++
>  include/linux/ve.h   |  1 +
>  3 files changed, 44 insertions(+), 5 deletions(-)
> 
> diff --git a/drivers/tty/pty.c b/drivers/tty/pty.c
> index f8610c77817a..2336204265ab 100644
> --- a/drivers/tty/pty.c
> +++ b/drivers/tty/pty.c
> @@ -628,6 +628,11 @@ bool vtty_is_master(struct tty_struct *tty)
>  	return tty->driver == vttym_driver;
>  }
>  
> +bool vtty_is_slave(struct tty_struct *tty)
> +{
> +	return tty->driver == vttys_driver;
> +}
> +
>  typedef struct {
>  	envid_t			veid;
>  	struct tty_struct	*vttys[MAX_NR_VTTY_CONSOLES];
> @@ -1082,15 +1087,31 @@ int vtty_open_master(envid_t veid, int idx)
>  		goto err_install;
>  	}
>  
> -	vtty_drop_context();
> -
>  	/*
> -	 * We're the master peer so increment
> -	 * slave counter as well.
> +	 * Serialize with a concurrent tty_release() on the vttys side.
> +	 * tty_lock guarantees we see the slave count after any in-flight
> +	 * close has decremented it.  If the slave has reached zero the
> +	 * pair is dying; return -EBUSY and let the caller retry; the
> +	 * old pair will be freed once we drop tty_mutex below, and the
> +	 * next attempt will create a fresh one.
> +	 *
> +	 * Incrementing slave->count under the same tty_lock prevents a
> +	 * concurrent tty_release() from seeing zero and starting the
> +	 * final-close path that would cause use-after-free.
>  	 */
> +	tty_lock(tty->link);
> +	if (tty->link->count == 0) {
> +		tty_unlock(tty->link);
> +		ret = -EBUSY;
> +		goto err_install;
> +	}
> +	tty->link->count++;
> +	tty_unlock(tty->link);
> +
> +	vtty_drop_context();
> +
>  	tty_add_file(tty, file);
>  	tty->count++;
> -	tty->link->count++;
>  	fd_install(fd, file);
>  	vtty_open(tty, file);
>  
> diff --git a/drivers/tty/tty_io.c b/drivers/tty/tty_io.c
> index 797a9d0ddd1e..507da79a24fa 100644
> --- a/drivers/tty/tty_io.c
> +++ b/drivers/tty/tty_io.c
> @@ -1864,6 +1864,23 @@ int tty_release(struct inode *inode, struct file *filp)
>  	/* check whether both sides are closing ... */
>  	final = !tty->count && !(o_tty && o_tty->count);
>  
> +#ifdef CONFIG_VE
> +	/*
> +	 * vtty: prevent double-final between concurrent master and slave close.
> +	 *
> +	 * For vtty slaves o_tty is NULL (PTY_TYPE_SLAVE), so the standard final
> +	 * check only looks at slave->count.
> +	 *
> +	 * When the master is closing concurrently, it holds tty_lock_slave(slave)
> +	 * while decrementing master->count, so while we hold tty_lock(slave),
> +	 * the master cannot have decremented yet - master->count is still > 0.
> +	 * If master->count is already 0 here, the master already returned
> +	 * from tty_release with final = false, therefore we are closing it.
> +	 */
> +	if (final && !o_tty && tty->link && vtty_is_slave(tty) && tty->link->count > 0)
> +		final = 0;
> +#endif
> +
>  	tty_unlock_slave(o_tty);
>  	tty_unlock(tty);
>  
> diff --git a/include/linux/ve.h b/include/linux/ve.h
> index b037f60225bb..bf5d9acb964c 100644
> --- a/include/linux/ve.h
> +++ b/include/linux/ve.h
> @@ -236,6 +236,7 @@ extern int vtty_open_master(envid_t veid, int idx);
>  extern void vtty_release(struct tty_struct *tty, struct tty_struct *o_tty,
>  			int *tty_closing, int *o_tty_closing);
>  extern bool vtty_is_master(struct tty_struct *tty);
> +extern bool vtty_is_slave(struct tty_struct *tty);
>  extern void vtty_alloc_tty_struct(const struct tty_driver *driver,
>  				  struct tty_struct *o_tty);
>  #endif /* CONFIG_TTY */